1. Field of the Invention
The invention relates to an ink jet recording method and an ink jet recording device that are favorably used for formation of a high-quality image at high speed.
2. Description of the Related Art
An ink jet method of ejecting ink in the form of liquid droplets from an ink ejector has been used in various kinds of printers for the reasons of being compact and less expensive, capable of forming an image without contacting a recording medium, and the like. Among these ink jet methods, there are a piezo ink jet method utilizing deformation of piezoelectric elements to eject ink and a thermal ink jet method utilizing a boiling phenomenon of ink due to thermal energy to eject the ink in the form of droplets, which have the characteristics of high resolution and high-speed printability.
Improvements in speed and image quality have currently become important objectives, upon printing by ejecting ink droplets onto a plain paper sheet or a non-water absorbing recording medium made of plastics or the like with an ink jet printer.
Ink jet recording is a method of ejecting ink droplets according to image data to form a line or an image on a recording medium with the liquid droplets. However, there have been problems in practical use, particularly in a case of recording on the above-described non-absorbing recording medium, e.g., bleeding of an image easily occurs, or mixing of adjacent ink droplets occurs on the recording medium to inhibit formation of a sharp image, when it takes time for the liquid droplets to dry or penetrate into the recording medium after having been ejected. When the liquid droplets mix with each other, the ejected adjacent liquid droplets coalesce with each other to shift from the positions at which they have landed, thereby causing unevenness in line width in a case of forming fine lines or unevenness in color in a case of forming a colored area, or the like. Further, since the degree of occurrence of unevenness in line width or color unevenness in a colored area varies depending on ink absorption and wettability of the surface of the recording medium, there has also been a problem that different images are formed on different types of recording media, even when the same ink is used under the same ejection conditions.
As a method of suppressing image bleeding or nonuniformity of line width, there is a method of promoting fixation of liquid droplets. For example, there have been disclosed methods of using inks of two-liquid type having reactivity and allowing them to react with each other on a recording medium to achieve a depicting property with high definition, such as a method of recording with ink containing an anionic dye after application of a liquid containing a basic polymer (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 63-60783), or a method of applying ink containing an anionic compound and a coloring material after application of a liquid composition containing a cationic substance (for example, refer to JP-A No. 8-174997).
An ink jet recording method has also been proposed in which an ultraviolet-curable ink is used as the ink, the ink dots ejected onto a recording medium are irradiated with an ultraviolet ray in conformity with the timing of ejection, then the dots are pre-cured to be thickened to such an extent that the adjacent dots do not mix with each other, and thereafter the dots are further irradiated with ultraviolet rays to be completely cured (for example, refer to JP-A No. 2004-42548).
Further, a method has been proposed that improves visibility or remedies bleeding of color ink, or a problem such as variation in the obtained images formed on different types of recording media, by applying a radiation curable white ink to form a uniform undercoating layer on a transparent or semi-transparent non-absorbing recording medium, then curing or thickening the layer by irradiating with radiation rays, and thereafter recording with a radiation curable color ink (for example, refer to JP-A No. 2003-145745 and JP-A No. 2004-42525). There has also been proposed a method in which a substantially transparent active ray-curable ink is applied onto a recording medium in place of the above radiation curable white ink by an ink jet head (for example, refer to JP-A No. 2005-96254).
However, in the method described in JP-A No. 2004-42548, although bleeding can be suppressed, there still remains a problem of variation in images among various types of recording media, and thus a problem of unevenness in line width, color or the like due to mixing of ink droplets has not been sufficiently solved. This problem of unevenness in line width, color or the like due to mixing of ink droplets has also not been sufficiently solved by either of the methods described in JP-A No. 2003-145745 or JP-A No. 2004-42525. Further, there sill remains a problem of unevenness in line width, color or the like due to mixing of ink droplets in the method described in JP-A No. 2005-96254.
On the other hand, when the ejected amount of an ink is small or the density of the ejected droplets is low, e.g., when an image is recorded by a head unit having low resolution in a single pass method by which an image can be formed at high speed, there are also problems in that disorders or bleeding in an image is caused by unregulated spreading of ink drops (dots), or to the contrary, white spots, degradation in density, bleeding or unevenness in an image is caused by insufficient spreading of the dots.
The invention has been made in view of the above problems and is intended to provide an ink jet recording method and an ink jet recording apparatus by which images having excellent uniformity can be recorded on various types of recording media and ink bleeding or unevenness in line width or color due to coalescence of ink droplets can be suppressed, and at the same time, an image can be well reproduced to details with high density while maintaining a uniform dot diameter, irrespective of the form of the image, when the image has low dot density such as low image resolution or image density and is recorded with a small amount of the ink.
The invention has been made based on the findings that it is important that the ejected and formed dots have the characteristics of spreading to a certain extent of area, and are capable of maintaining the shape thereof when the dots spread to coalesce with each other, in order to retain a high degree of density over the whole area of the image to details and allow reproduction of a clear image, in a case where the amount of the ink ejected at the time of recording is small and the dot density is relatively low.
When the dots spread to a desired extent, and the shapes thereof are maintained when they coalesce with each other, a certain degree of density can be achieved even when an image is recorded by a single pass method with a low-cost apparatus provided with a head unit having low resolution, and also the quality of a recorded image can be improved.